We need to look no further than nature to find inspiration for many of the technologies we work on today. One such field that observations on natural systems have impacted significantly in the recent years is adhesive technologies. While conventional adhesives rely on very soft materials or viscous liquids, nature offers a unique system composed of adhesive elements made of relatively rigid materials. These adhesive elements are comprised of millions of tiny fibers varying in size and geometrical complexity depending on the animal that bears them. Some insects, spiders, and anoles have fibers with effective diameters in the order of microns. Other animals such as the gecko lizard bear micro-scale stalks which branch down to nano-scale fibers forming intricate hierarchical structures. The common aspect of fibrillar structuring is its ability to conform to the adhering surface, improve contact area and create an attractive force between individual fibers and the surface. The cumulative effect from the adhesion contribution of every fiber in contact is capable of generating adhesive strengths up to 100 kPa as measured in the case of the gecko lizard.
A great deal of research has been performed to analyze the structure of natural fibrillar adhesives and measure their performance, understand the main principles of enhanced adhesion, and fabricate synthetic counterparts of biological fiber adhesives.
A common aspect of natural fibers among species, which is of interest in this work, is that the cross section of a natural fiber is rarely constant along its length. It gradually increases close to its terminal end forming what is referred to as mushroom-shaped fibers. While initial fabrication attempts for synthetic adhesives were limited to constant cross section cylindrical fibers, realization of the actual shape of natural fibers has led to synthetic fibers with mushroom tips. Adhesives comprised of mushroom-shaped fibers have shown significant improvements over cylindrical fibers. Furthermore, measured adhesive strengths have matched and in some instances such as smooth surface applications surpassed adhesive strengths recorded for gecko footpads.
The force required to detach a mushroom-shaped fiber is greater than that of a cylindrical fiber because the contact area for a mushroom-shaped fiber is larger. Work by del Campo et al., Del Campo A., Greiner C., Arzt E. Langmuir 2007, 23, 10235-10243, reports enhancements in pull-off loads as much as 40-fold with mushroom-shaped fibers over cylindrical fibers of equal height and stalk radius. Interestingly, for mushroom-shaped fibers which exhibited this enhancement, the contact area is only 1.7 times the contact area of flat tip cylindrical fibers. This fact points to the existence of an adhesion enhancement mechanism other than just the increase in contact area with mushroom-shaped fibers.
In this invention, the pull-off stress of mushroom-shaped fibers using a cohesive zone model and finite elements (FE) simulations is examined. This model is then used to determine the optimal parameters for maximum pull-off stress. Two parameters are identified for design and optimization: the edge angle of the fiber tip γ and the ratio of the radius of the tip to the radius of the stalk β. In addition, the impact of the shape of the edge tip—where the surface and sides of the mushroom tip intersect—is evaluated and a preferred shape is identified.
While previous literature has described that microfibers with mushroom-shaped tips demonstrate enhanced adhesion when compared with cylindrical microfibers, there exists no understanding of what geometric parameters result in microfibers with optimized adhesion.